There is an increasing demand for simple, self-controlling, speed-sensing limited-slip devices having locking characteristics and high torque capacity over a given set of conditions, in particular in Sport Utility Vehicle and light truck axle and transfer case applications. Existing speed-sensing systems have a degressive locking characteristic; some designs are too complex for high volume production. More recent designs of speed sensing limited-slip devices have high-torque progressive engagement and are tuned to minimize driveline windup. One type of a limited-slip device is a limited-slip differential. Progressive types of limited-slip differentials allow for a wide range of calibration in specifying the level of torque transfer at any particular speed difference. This self-controlling stand-alone limited-slip differential type system consists of a shear pump design creating a pressure proportional to a speed difference, which engages a friction clutch to transmit the torque through the drive system. A feature of the shear pump is that it is self-contained and independent, requiring no external source of hydraulic fluid. The viscous shear pump, which is filled, typically, with silicone fluid, provides significant benefits in packaging, temperature stability, durability, and weight. However, the hydraulic fluid undergoes a decrease in viscosity over the operating temperature. When silicone is used as the operating hydraulic fluid, its viscosity decreases linearly as the temperature increases causing a reduction in pump pressure output; decreasing the torque transferring capabilities of the speed sensing limited-slip differential. Also, the torque transfer efficiency, as a function of slip speed, is reduced over the operating temperature by the decrease in viscosity of the hydraulic fluid.
The speed sensing limited-slip differential is comprised of two distinct functional parts: the shear pump and the controlled multi-plate wet clutch. The ability to separate the controlling function from the locking function provides significant flexibility in specifying the torque characteristic at any particular speed difference. The wet clutch provides high power density and reliability for torque transfer. The design of the self-contained viscous shear pump generates a pressure proportional to a speed difference, which engages the friction clutch via a pressure piston to transmit the torque.
The pressure generation in the shear pump is based on shearing a high viscosity silicone fluid in a laterally sealed shear channel. The shear channel consists of a pumping groove located in a plate and a flat surface of a second plate with relative movement to one another. This pump channel is filled with a hydraulic fluid having high viscosity properties, such as silicone fluid. One surface pulls the viscous fluid through the shear channel by the relative speed direction from the beginning of the sealed channel (suction side) to the end (pressure side). With the suction side connected to a reservoir and the pressure side to a pressure chamber that exerts force upon a piston, the shear pump generates a fluid flow from the reservoir to the piston. The generated pressure and fluid volume flow is approximately proportional to the relative speed and is a function of fluid viscosity and geometry of the shear channel. Transferring this linear model into a rotating system, the second plate becomes a simple disc (the feed disc) fixed to the hub and the first plate with the channel becomes a grooved disc (the pump disc) fixed to the housing. The hub drives the feed disc rotatably relative to the pump disc. The pump disc includes a circumferential pumping groove and connecting holes forming the shear channel in conjunction with the feed disc end face. The shear pump is covered by a pressure piston whereas the reservoir is covered by a spring-loaded compensation piston on the opposite side of the housing or cover.
The fluid is drawn out from the reservoir via the connecting hole in the pump disc, then moved through the channel due to the shear forces when the feed disc rotates relative to the pump disc, and directed between feed disc and pressure piston in the pressure chamber. The generated pressure acts upon the pressure piston in a limited-slip differential application by forcing the pressure piston against a friction clutch as well as forcing the feed disc against the pump disc assuring a tight seal. Due to this self-sealing effect, there is no need for a complex sealing design.
Single directional operation of the shear pump occurs when the pump disc is non-rotatable relative to the housing or cover of the limited-slip differential. To provide the shear pump with bi-directional operation, an additional control function is used that allows correct orientation of the reservoir to the suction side of the pump disc in either rotational direction. This is accomplished by allowing the pump disc to have two rotational positions in the housing (indexable). Depending on the slip speed direction, the pump disc automatically indexes, thus allowing the suction side and outlet side of the pump disc to be correctly aligned to the reservoir and pressure chamber. The switching function also enables an asymmetric pressure characteristic for both slip speed directions to be built into the limited-slip differential if required.
A feature of the shear pump is that it is self-contained and independent, requiring no external source of viscous fluid or servicing. The system can be internally or externally mounted and provides significant benefits in packaging, durability, and weight compared to conventional vane type or gerotor type pump systems. The pump is filled with a hydraulic fluid, typically, silicone fluid. The physical properties of the silicone fluid provide superior temperature stability, minimal temperature sensitivity, and excellent durability. However, one fluid property weakness of silicone fluid is that its viscosity is linearly related and decreases with increasing temperature. Another feature of the shear pump, is that the pump pressure characteristic, in general, is linear versus the speed difference and can be tuned by the pump disc geometry and by selecting the silicone fluid having a particular viscosity. However, there is no compensating or tuning capability in the current art that can offset or efficiently control the decreasing viscosity of the viscous fluid as the temperature increases.
The generated fluid flow of the shear pump is a function of the pump design, the pump pressure and the slip speed. The fluid flow rate of the shear pump decreases with increasing pump pressure and tends to zero when the pressure reaches its maximum. Independent of the slip speed acceleration, the pump pressure always approaches its specific maximum in an asymptotic manner. This unique feature of the shear pump guarantees a smooth torque engagement without any torque peaks. When the slip is reduced, the pressure applied is quickly released via the shear channel into the reservoir.
Conventional gear or vane type pumps generate a pressure with a certain periodical fluctuation versus one revolution. This may cause a pulsating bias torque which can excite torsional vibrations and noises, especially when used in a driveline. Due to the new working principle, the pressure generated in the shear pump is constant without any fluctuation particularly at very low slip speeds. The shear pump as a self-controlling stand-alone device can be applied as a limited-slip differential in front and rear axles as well as in center differentials, or directly as an “on demand” torque transmission between the axles of a 4WD vehicle. The shear pump can be used in other applications, especially so where continuous non-pulsating pressure and or flow requirement as a function of pump velocity are of interest. However, all shear pump applications are subject to the decreasing performance effect caused by the temperature related viscosity of the hydraulic fluid.
Therefore, there is a need for a temperature compensated shear pump that will mitigate the temperature related viscosity change in the fluid and further enhance shear pump performance.